1. Field of the Invention
This invention relates to a process and apparatus for catalytically reducing noxious substances in flue gas.
2. Description of the Related Art
German Pat. No. 3,536,958 which corresponds to U.S. application Ser. No. 06/919,708 (hereinafter referred to as the "prior Application" and the disclosure of which is herein incorporated by reference), now U.S. Pat. No. 4,756,891 relates to a method and apparatus for preheating combustion air and for catalytically reducing noxious substances in flue gas. The essential purpose of the invention disclosed therein is to prevent a decrease in catalytic activity during operation and to additionally reduce the space requirement of such systems by combining the catalyst and the preheater for the combustion air.
After leaving the apparatus, the flue gas is introduced into further purification stages, such as an electrofiltering apparatus and a flue gas desulfurization system. However, it is also known to perform flue gas desulfurization before catalytic reduction of the noxious substances as disclosed in German Published Application No. 3,407,277. This substantially protects the sensitive catalyst from sulfur compounds. To keep the required external energy for reheating the flue gas as low as possible, however, a regenerative gas preheater is connected in the circuit. Leakages from this regenerative gas preheater are known to adversely affect the efficiency of the catalyst and, thus, to require a corresponding enlargement of the quantity of catalyst because this preheater effects polluting the cleaned flue gas with uncleaned flue gas.